In the electrolytic process for making alkali metal hydroxide and chlorine by electrolytic decomposition of alkali metal chloride in electrolytic cells, of both the diaphragm as well as the mercury type, the chlorine gas emerging from the cell is collected in a manifold under slight vacuum and ordinarily contains, besides water vapor and brine spray, small amounts of hydrogen from diaphragm diffusion or from amalgam-anolyte reactions, as well as small amounts of air sucked in through leakage in the equipment. Typically, the gas as obtained from the cells contains in the order of about 96 percent (by volume) of chlorine, 1 percent of hydrogen, and 3 percent of air (inerts). Often, additional air is introduced into the cell gas to maintain the hydrogen concentration in the tail gas from the following chlorine liquefaction operation below explosive limits, say below about 6 volume percent. After washing and drying, usually with sulfuric acid, the cell gas is compressed and cooled in a condenser to effect liquefaction of the chlorine. The unliquefied tail gas from the chlorine condenser, which ordinarily may comprise up to about 10 percent of the chlorine generated in the cells, contains enriched amounts of hydrogen and air. A typical tail gas composition may be, by volume: chlorine, 30 percent; air, 64 percent; and hydrogen, 6 percent. This tail gas cannot safely be compressed to higher pressure to condense additional amounts of chlorine because explosive conditions would develop due to increasing hydrogen concentration in the remaining tail gas as chlorine is condensed therefrom. Ordinarily, such tail gas is disposed of in the manufacture of by-products, such as hypochlorite bleach and chlorinated hydrocarbons. However, markets for such by-products do not always develop, and it would be desirable to recover additional amounts of chlorine from such tail gas.
The present invention has as its object selective removal of hydrogen from hydrogen-contaminated chlorine gas, such as the cell gas from the chlorine/caustic process, or the tail gas from liquefaction of chlorine from such cell gas. Tail gas from which hydrogen has been removed can thereafter be safely recompressed for recovery of additional liquefied chlorine.